Image processing method to reduce marking material coverage in printing processes

ABSTRACT

A method for processing a color image for printing reduces an amount of marking material used in the printed image. The color image includes multiple separations, including at least one non-black color separation and a black color separation, each defined by a bitmap of single bit pixels each having a pixel value and arranged in an array of scan lines. The method includes the steps of, for each separation bitmap, measuring a marking material coverage value over a given area within the image; summing the marking material coverage of each separation bitmap to determine measured marking material coverage; comparing desired marking material coverage with measured total marking material coverage to derive an ON/OFF ratio, where the ON/OFF ratio represents a number of pixels to be turned OFF, compared to the number of pixels in a separation; setting a number of pixels to OFF in the non-black color separation bitmaps, in a larger number of pixels in said separation, in accordance with the turn OFF ratio; retaining an ON/OFF state of all of the pixels in the black color separation irrespective of the ON/OFF ratio; and outputting the separation bitmaps to a print controller. The method provides high quality printed images without causing an undesired color shift in the image due to the coverage reduction process.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to an image processing method for reducing theamount of marking material required to print a colored image, to avoidproblems common to the use of excessive amounts of ink. In particular,this invention relates to an image processing method in which the colorseparations of an image (such as cyan, magenta and yellow) are processedthrough a coverage reduction process, while the black color separationis passed unchanged.

2. Description of Related Art

Full color printing has become a desired goal of office, as well ashome, products. One type of full color printer that has significantpotential for fulfilling such a goal is the ink jet printer, dueprimarily to its low purchase costs combined with its high qualityoutput. In one common design of such printers, a reservoir of liquid inkis connected to an ink output orifice via a capillary tube. In the tube,a heater element is provided, responsive to an on/off or binary printingsignal. When printing is required and a printing signal is directed tothe heater element, the heater element rapidly heats ink in thecapillary tube adjacent thereto to a gaseous state, producing a pressuredifferential that expels a droplet of ink from the orifice, directingthe droplet to a sheet of receiving material, such as paper. Colorprinting is accomplished by providing multiple layers or separations ofink on the page. Commonly, colors are provided by subtractivecombinations of cyan, magenta and yellow inks. To print black, acombination of equal amounts of cyan, magenta and yellow is printed, ora fourth black ink is used as a substitute. Undercolor removal, a wellknown process in the printing arts, can be used to print a single layerof black ink as a substitute for the combination of equal amounts ofcyan, magenta and yellow. For a fuller discussion of under color removaland its application to electronically derived or created images,reference is made to J. A. C. Yule, Principles of Color Reproduction,(John Wiley & Sons, Inc., New York, 1967), pages 294-327. Other fullcolor printing processes may use dry powder or liquid toners.

A problem of ink jet printers is that the liquid inks used have a finitedrying time, which tends to be somewhat longer than desirable. Further,the drying time of any particular area is at least partly a function ofthe amount of ink deposited on that area. That is, as the amount of inkdeposited in a given area increases, the drying time of the ink in thatarea also generally increases. While satisfactory drying times arepossible with black-only or single separation printing, once multipleseparations are required, the large amount of liquid on the page causesthe problems of ink puddling or pooling, bleeding to adjacent imageareas, and flow through to the back side of the receiving material.Paper cockle is also a problem due to saturation of the paper receivingmaterial and subsequent rapid drying. Particularly, problems are notedin the printed image at high ink coverage areas, and high coverage areaswhere high contrast image edges occur. While certain materialsvariations, such as selection of different inks or the use of specialpapers, may resolve some of these problems, each brings its own distinctproblems to the process. While special treated papers optimized for inkjet use are possible, plain papers are preferred from cost andconvenience standpoints.

Using one available set of materials, a maximum ink coverage of about150% is required for printing without artifacts resulting from too muchink. As used herein, ink coverage refers to the number of ON pixels in aregion for all the separations, divided by the total number of pixels inthe region in one separation. Without undercolor removal, a typical fullcolor image may require ink coverage in the range of 200-300%. Withundercolor removal, maximum ink coverage may be down to 200%, but nolower. Additionally, it would be highly desirable for the process tooperate irrespective of image content, or on the separation binarybitmaps without further image information.

While ink jet printing has a notable problem with the case of high inkcoverage, other printing processes that operate on a pixel by pixelbasis also have problems with excessive marking material. Notably,electrophotographic printing methods using a pixel by pixel printingprocess for forming a latent image for development by dry or liquidtoner marking materials, can suffer from excessive marking material,evidenced by sheet cockling, and curling caused by differentialshrinkage of toner and paper in the printing process.

U.S. Pat. No. 4,930,018 to Chan et al. teaches the reduction of papercockle and graininess of ink jet prints. Printing of a given scan lineoccurs multiple times, with three different dye loadings, with pixelsrequiring the highest dye loading printed on one pass, pixels requiringan intermediate dye loading printed on another pass, and pixelsrequiring the lowest dye loading on another pass. The method takes asinput continuous tone RGB (red-green-blue) images and performs RGB-CMYK(cyan-magenta-yellow key or black) conversion with full under colorremoval. As understood, printing is performed at half resolution, sothat “pixels” in the input image correspond to 2×2 blocks in the outputimage. The image data is first error diffused from 8 bits per pixel perseparation to 4 bits per pixel per separation. Then, for each pixel, acount of up to 4 drops of each dye loading is computed, for eachseparation. There are multiple choices, ranked in order of total inkcoverage. If the highest coverage choice exceeds the maximum allowablecoverage, the separation with highest coverage is changed to use a lowercoverage value for the same gray level, if possible. If it is notpossible to stay at the same gray level, the gray level for thatseparation is dropped by one, and the error passed on to neighbors. Theprocess iterates until the total ink coverage is as low as required.Pixels within the 2×2 block are assigned values (0 or 1) by proceedingaround the block in clockwise order, and filling in pixels in order.First, the high dye load pixels are turned on, then the medium, then thelow. Within each dye loading group, first black is turned on, untilthere are no more black pixels of that dye loading, then the next pixelsin the cycle are turned on, until there are no more cyan required, thenmagenta, and yellow, and then the next dye load group. By maximizing inkcoverage and using multiple dye loadings, they reduce the noisiness ofthe image, and by maintaining the total ink coverage within knownlimits, they prevent the many problems associated with excessive ink.

U.S. Pat. No. 4,999,646 to Trask teaches limiting coverage to 100%coverage (by the above definition of coverage), or perhaps between 100and 200% coverage (if 100% corresponds exactly to no white spaces on apage), owing to the circular shape and overlap of print dots. Coverageis limited by using 2×2 super pixels and assigning each one drop perpixel in a combination that depends on the color required. Assuming onebit per separation input with fall undercolor removal, there are eightpossible colors that could be(requested (including white). In order toreduce patterning due to the multiple swaths, two passes are used, eachof a checkerboard pattern of pixels (the two passes being offset toprovide full coverage). The two pass process allows ink to dry betweenpasses.

U.S. Pat. No. 5,237,344 to Tasaki et al. describe a method for reducingthe amount of ink printed to 50%, 75% or 66%. The method uses fixedpatterns of turn-off locations (e.g., a checkerboard for 50%) andselects the pattern based on the printing mode (reverse character mode,block graphic mode or normal character mode), the character selected,and possibly the relative humidity. Apparently, the method is designedfor single color (black) printing: if it were used for multipleseparation (e.g., red formed from yellow and magenta) printing, bothseparations would be turned off in the same place, resulting in moreobvious patterns. The small set of fixed turn-off patterns makes themethod very sensitive to line angle, as lines at some angles will havemore pixels turned off than others. Also the method is only useful forcharacters from a built-in font, including graphic characters; arbitraryfonts and shapes, such as are requested in documents created usingindustry standard page description languages, such as PCL or PostScript,cannot be handled in this way.

U.S. Pat. No. 5,515,479 to Klassen, the entire disclosure of which isincorporated herein by reference, discloses a coverage reduction methodfor processing images for printing. The method includes the steps of,for each separation, making a pixel level determination of markingmaterial coverage, based on the number of pixels within a given areathat are turned on. If a determination is made that too much markingmaterial will be placed within the given area, a fraction of pixels inthe area are turned off to reduce the amount of marking material thatwill be used for the given area. The method uses a processing orderthrough each given area to prevent artifacts from occurring in the pixelreduction step, which tends to randomize the turn off effect.Additionally, in the disclosed method, the results of the determinationare compared for each separation, to determine that the turnoff resultallows at least one corresponding pixel among each of the separations tobe turned on.

Other coverage reduction methods are also described in U.S. Pat. Nos.5,649,071, 5,635,967, 5,563,985, and 5,519,815, the entire disclosuresof which are incorporated herein by reference. U.S. Pat. No. 5,649,071to Klassen et al. discloses a method of processing images preparatory toprinting in a color printer adapted to printing serial colorseparations, that when superposed form the final color image. In thedisclosed method, separation color images are received for printingdefined by continuous tone separation signals, which are color correctedand halftoned preparatory to printing the color image. The receivedcontinuous tone separation image signals are used to derive an estimateof marking material coverage. The marking material coverage estimate issubsequently used in association with random number generation to turnOFF otherwise ON pixels in each separation as the separation is printedfor the reduction of marking material coverage.

U.S. Pat. No. 5,635,967 to Klassen discloses a method and apparatus forreducing marking material coverage in reproduction of edges in ahalftoned image. The method includes the steps of: a) detecting edges inthe image using edge filtering; and b) reducing marking materialcoverage at the detected edges. Interseparation correlation may also beprovided, for the purpose of ascertaining whether a detected edge isagainst a white or no colorant region, in which circumstance, markingmaterial coverage is not reduced. The patent further describes thereduction of marking material coverage at the detected edges inproportion to the overall coverage in the image region, taking intoaccount the kind of colors (primary or secondary) forming the edge.

U.S. Pat. No. 5,563,985 to Klassen et al. discloses a method forprocessing images for printing coverage reduced images. The methodincludes the steps, for each separation, of making a pixel leveldetermination of marking material coverage, based on the number ofpixels within a given area that are turned on. If a determination ismade that too much marking material will be placed within the givenarea, a random number function is used to turn off a fraction of pixelsin the area to reduce the amount of marking material that will be usedfor the given area. To prevent artifacts from occurring in the pixelreduction step, a processing order through each given area is used thattends to randomize the turn off effect. Additionally, the results of thedetermination are compared for each separation, to determine that theturnoff result allows at least one corresponding pixel among each of theseparations to be turned ON.

Finally, U.S. Pat. No. 5,519,815 to Klassen discloses a method anddevice for processing color images preparatory to printing to effectcoverage reduction. The method of reducing marking material coverage intext and line art areas of secondary colors includes the steps of: a)determining locations of text and line art color pixels having excessivemarking material coverage; b) upon determining the locations of colorpixels having excessive marking material coverage, processing separationpixels to turn OFF a predetermined portion of the separation pixelscorresponding to color pixels having excessive marking materialcoverage; and c) to prevent artifacts from occurring in the pixelreduction step, processing a given area of separation pixels in an orderwhich tends to randomize the turn OFF effect.

SUMMARY OF THE INVENTION

Despite these various coverage reduction techniques, a need exists forimproved coverage reduction processes that provide high quality printedimages with little image degradation.

In this respect, the present inventor has discovered that when all ofthe color separations are processed through a coverage reductionprocess, image shifts may occur, leading to reduced image quality. Theappearance change occurs largely in an effect of producing lesssaturated or “duller” looking copy. However, the inventors unexpectedlydiscovered that if the black color separation is retained unchanged,while the other color separations (such as cyan, magenta and yellow) arecoverage reduction processed, a resultant image is obtained that doesnot suffer from such color shift problems, and which more closelymatches the color scheme of the original image.

Thus, in embodiments of this invention, the present invention provides amethod for processing a color image for printing to reduce an amount ofmarking material used therefor, the color image comprised of multipleseparations, comprising at least one non-black color separation and ablack color separation, each defined by a bitmap of single bit pixelseach having a pixel value and arranged in an array of scan lines. Themethod generally comprises the steps of:

for each separation bitmap, measuring a marking material coverage valueover a given area within the image;

summing the marking material coverage of each separation bitmap todetermine measured marking material coverage;

comparing desired marking material coverage with measured total markingmaterial coverage to derive an ON/OFF ratio, where the ON/OFF ratiorepresents a number of pixels to be turned OFF, irrespective of theirpixel value, compared to the number of pixels initially ON in aseparation;

setting a number of pixels to OFF in the non-black color separationbitmaps, in a larger number of pixels in said separation, in accordancewith the turn OFF ratio;

retaining an ON/OFF state of all of the pixels in the black colorseparation irrespective of the ON/OFF ratio; and

outputting the separation bitmaps to a print controller.

The coverage reduction method of the present invention can be applied toink jet printers, as well as to other printers that print an image on apixel-by-pixel basis.

The present invention also provides a device for processing a colorimage, which device effects coverage reduction of the image.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages and features of this invention will beapparent from the following, especially when considered with theaccompanying drawings, in which:

FIG. 1 is a functional block diagram of a system for carrying out thepresent invention;

FIG. 2 is an example of the processing pattern;

FIG. 3 shows a flow chart demonstrating the overall inventive method;

FIG. 4 shows a flow chart of the method of determining ink coverage;

FIG. 5 shows a flow chart of the method of determining whether a pixelshould be designated as OFF; and

FIG. 6 shows the separate step of the method of white pixel prevention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A basic functional block diagram for a system for carrying out thepresent invention is shown in FIG. 1. As shown therein, a plurality ofvideo inputs 10 c, 10 y, 10 m, and 10 k, each representing one of thecyan, magenta, yellow or black separations, respectively (B N (x,y)) ofimage B(x,y) are directed into the ink coverage reduction system.However, although FIG. 1 shows the bitmap for the black color separationbeing processed, it is important to note that according to the presentinvention, the black color separation is passed unchanged through thecoverage reduction process. This aspect of the invention will becomeclear in reference to the further Figures, described in more detailbelow. Because the process of the system is mostly redundant for eachseparation, initially only the cyan bitmap B C (x,y) will be described.Bitmap B C (x,y) is a binary bitmap in this system, defined along thefast scan (x-axis) and slow scan (y-axis) axes, describing the image interms of 1 bit pixels arrayed in scan lines. Each pixel is an imagesignal, which may be said to have a condition or image state, whichdescribes for a binary pixel whether a print dot will be made at theoutput or not. This may also be equated to whether the pixel is ON orOFF, i.e., 1 or 0. Thus, there are two image states for any pixel in abinary printer. For a multilevel printer, there may be more than twopixel values, and a similarly larger number of pixel image states.

To determine ink coverage over a given area, bitmap B C (x,y) isdirected initially to a one byte (8 bit) latch 12, which stores eightone bit pixel values in order 1 to 8. This 8 bit stored binary string,which represents the bitmap image in an 8 pixel×1 scan line portion ofthe given area, is used as an address to access a 256 entry look uptable (LUT) 14. The data stored at each of addresses 16 in the look uptable 14 represents ink coverage values 18, for the 8 pixel area storedat latch 12. It will no doubt be appreciated that many of these inkcoverage values are the same, as shown by the demonstration of address00000001 and address 00000010, which each have ink coverage values of 1.Should ink coverage be non-linear in character, because, for example,the ink jet device tends to deposit more or less ink when operated incertain image formations, then such non-linearity may be accounted forin loading the LUT 14 values.

Ink coverage values 18, which in the example can be represented by 3 bitbinary values, now form a signal or function I C (x,y), representingpredicted ink coverage in an 8 pixel×1 scan line. A group of thesevalues, stored in 8 row memory 20, are summed together at accumulator 22to derive total ink coverage over a given area, in this case, 8 pixels×8scan lines in size. The output of accumulator 22 is a value p, thepredicted or measured ink coverage value for each separation of thegiven area of the image. The values of p (p_(c), p_(m), p_(y), p_(k)),measured for each of the cyan, magenta, yellow and black separationbitmaps, are added to derive values m=p_(c)+p_(m)+p_(y)+p_(k),mc=p_(c)+p_(m)+p_(y) and m_(k)=p_(k), which are the predicted ormeasured ink coverage value for the given area of the image. Here, m isthe total predicted or measured ink coverage value for all of the colorseparations; m. is the predicted or measured ink coverage value for thecolored separations cyan, magenta and yellow; and m_(k) is the predictedor measured ink coverage value for the black separation. Thus,m=m_(c)+m_(k). It is possible that some colors bleed more than others;if so, some separations might be weighted more heavily in the summationin embodiments of the present invention.

An 8 pixel by 8 scan line block for the given area has been selected forprocessing convenience, and because it reflects a useful size for theink reduction determination. If the area is too large, the ink coveragevariations will be averaged in the reduction determination, giving lowink coverage indications when portions within the large area exceed themaximum desirable ink coverage. The choice of area or window size is acompromise between the precision with which coverage can be measured(the larger the area the more possible ink coverage levels) and theaccuracy of its measurement. One way of increasing the precision withoutreducing the locality is to use a weighted sum (more heavily weightingthe center) to find the coverage near the center of the window. Thiswould work best for finding the coverage in the neighborhood of a singlepixel. In this embodiment, coverage was determined using an unweightedaverage over the 8×8 area, as it is less expensive to compute. Finally,there is very little difference between using an 8×8 window and, forexample a 6×6 window. Of course, many possible areas of different sizescan be used. The 8×8 window allows for greater efficiency ofimplementation as it uses complete bytes. Computing coverage in an 8×8window, always aligned on a 4 bit boundary can be done efficiently usingsmall tables. When the window is byte-aligned, a 256 entry table may beindexed by the contents of the scanline covered by the window, for eachscanline in the window, summing the results; when it is notbyte-aligned, two 16-element tables for may be indexed for eachscanline, to compute:

cl(p1&Oxf)+cr(p2>>4),

where p1 and p2 are the values in the image beneath the window; and cland cr are appropriately initialized tables having 16 entries giving bitcounts.

This expression refers to the use of the low order four bits of p1 toindex into table cl, and the high order bits of p2 are used to indexinto table cr.

Values m, m_(c), and m_(k) are used at ON/OFF logic 24 to apply the turnoff ratio to each separation of the image except for the blackseparation. Here, the desired turn off ratio is represented generally byd/m, where d represents the maximum desired number of pixels ON in anarea, m represents the predicted or measured total ink coverage value,and d/m therefore represent ratio reflecting that d pixels out of everym pixels in the separation bitmap should be left on. However, asdescribed above, in the present invention the black separation is passedthrough the coverage reduction phase (turn off logic) unchanged. Thus,based on the measured values of m, m_(c), and m_(k), computed above, itis apparent that the portion of d corresponding to the black separation,i.e., d_(k), must be left equal to m_(k). Thus, the portion of dcorresponding to the remaining color separations (cyan, magenta andyellow), which will be represented by d_(c), will be the remaining partof d, and can be represented as d_(c)=d−d_(k)=d−m_(k).

In the color depletion phase, the turn off logic operates on the colorseparations (i.e., all of the separations except for black) to turn offpixels to effect coverage reduction. Although the following descriptionis generic to any color separation, it is important to note that onlythe cyan, magenta, and yellow separations are processed. Thus, in agiven print operation, the turn off logic 24 accepts two input values.The value d is the desired print coverage, which can be fixed for agiven printer, or can be varied (either by input or hardware change)based on particular combinations of inks, paper types, and printingapplications. The second input, or set of inputs, is m, m_(c), andm_(k), as described above. Because the black color separation is passedunchanged through the turn off logic, the total number of pixels left ONafter processing the portion of the image can be represented asd_(c)/m_(c).

In embodiments of the present invention, it may be convenient to comparethe appropriate ratios d/m to 1, where a ratio greater than 1 indicatesno need to turn off any pixels. It will be assumed hereinafter that sucha comparison has taken place and has indicated a value of d/m less than1, indicating that at least some of the pixels need to be turned OFF. Inone implementation, ON/OFF logic 24 may include a counter, which countsfrom 0 to m_(c) by increments of d_(c) for each pixel on in B C (x,y).Upon reaching or exceeding m_(c), an output representing an ON signal isproduced, to indicate that the current pixel should be left on. Thiswill be further illustrated hereinafter.

There are many possible ways to determine ink coverage for an image, andparticularly for binary printers. For binary printers, the presence orabsence of a print dot in each separation can be taken as a predictedamount of ink. By summing all the print dots in an area small enough tobe useful in the ink reduction determination, a predicted value of inkcoverage can be made. In a multilevel or gray level printer, themeasured amount of ink is a function of the gray level value of thepixel, or the percentage of the maximum ink value deposited at a pixelin the image.

The requirement that regions that need no change receive no change ismet by never turning ON pixels that were originally OFF, and onlyturning OFF pixels where the coverage is too high.

As applied to cyan bitmap B C (x,y), the “turn OFF ratio” or ON/OFFratio of d/m (or d_(c)/m_(c)) would result in a periodic pattern, which,when imposed on a halftone pattern probably existing in B C (x,y), wouldresult in undesirable moire patterns in the reproduction of the image.Accordingly, one method of alleviating this problem is to randomize theapplication of the turn OFF ratio to the bitmap. However, it is alsodesirable that the determination of ON/OFF be made with knowledge of thedetermination for adjacent pixels, so that the application of the turnOFF ratio can be applied uniformly through the given area. Accordingly,and with reference to FIGS. 2a-e, where FIG. 2a shows the usual methodof progressing through a bitmap, processing pixels 1 to n, from scanline 1 to scan line k, FIGS. 2b-e show other processing orders (shownfor only 4×4 areas, as will be explained below), with a preferredprocessing order being the area filling curve of FIG. 2e. It will nodoubt be appreciated that other processing orders are possible, whichprocess pixels in an order that is relatively random with respect tocommon halftone patterns, and retains dependence on adjacent pixels.Ordering of pixels is accomplished by storing a 4×4 set of pixels forwhich m has been determined in memory 25, and subsequently reading thevalues of B C (x,y) out of memory with processing order control 26,which reorders pixels of B C (x,y) in accordance with the selected pixelorder function.

Once the turn OFF ratio d_(c)/m_(c) for an 8×8 set of pixels isdetermined, that ratio is applied to a 4×4 pixel grouping therewithin,processed in accordance with the process ordering of FIGS. 2b-e. The useof an 8×8 area for determining the turn OFF ratio smoothes abruptchanges, which might occur if a smaller area is used for thatdetermination.

At ON/OFF logic 24, a logical process is employed implementing thedetermined turn OFF ratio. The fraction of pixels to be left ON is theratio of the desired coverage to the measured coverage. This can beexactly retained as a rational number, d/m. A simple logic processensuring that d of every m pixels are turned on is:

f=0 for each pixel

f=f+d_(c)/m_(c) if(f≧1) turn on this pixel

f=f−1

Next pixel

Where f, initially set to zero, is the (fractional) number of pixelsthat should have been turned on since the last one was turned on, oralternatively the fraction of the way to the next pixel that should beturned on.

Pixels are spaced as uniformly as possible along the path that gives theorder in which they are visited. If the control variable f isinitialized only at the start of the page, and not in each window, theaverage over a larger region will be correct. By scaling f by m_(c), theuse of non-integer values is eliminated:

f=0 for each pixel

f=f+d_(c) if(f≧m_(c)) turn on this pixel

f=f−1

Next pixel

The initial setting of f to zero is arbitrary, as over an entire imagethe initial value of f is of no consequence. To reduce inter-separationcorrelations it may be advantageous to use various values between 0 andm_(c) for the initial settings of the control variable f for differentseparations. The comparison with mc is also arbitrary, for the samereason. If the comparison is with 0 it may be more efficient and can beaccomplished without changing the result in any way by subtracting m_(c)from the initial value of f:

f=−m_(c) for each pixel

f=f+d_(c) if(f≧0) turn on this pixel

f=f−1

Next pixel

It will be noted that this logic is similar to Bresenham's algorithm fordrawing lines, which for computer graphics determines the distance alonga first axis, before a step must be made in the second axis for a lineextending in two axes.

At block 38, the function is reordered into the original image order asB C ON/OFF (x,y), which is a mask representing the turn OFF function. Atblock 40, white pixel prevention logic is provided, to prevent theoccurrence of a corresponding pixel in every separation from beingturned OFF, which might allow white or background show through. Assuming100% coverage is acceptable, totally white pixels should not beintroduced into the image. Thus, if a full color pixel has 200%coverage, and a pixel in one separation is turned OFF, the correspondingpixels in other separations should not all be turned OFF when that pixelis visited for the other separation:

fc=0

fm=m/4

fy=m/2

fk=3m/4

calculate the pixelwise coverage for each pixel/separation pair, in someorder

if this pixel is ON in this separation fsep=fsep+d

if(fsep≧m) or the coverage in this pixel is 1 turn ON this pixel

fsep=fsep−m else

turn OFF this pixel

decrement the coverage in this pixel by 1

Accordingly, a set of ON/OFF conditions is created for a 4×4 area of theimage, reducing the number of ON pixels in that area in accordance withthe determining turn OFF ratio, B′ C ON/OFF (x,y). This set of values isthen used as an input to a masking logic 42, which uses B′ C ON/OFF(x,y) in altering the bitmap B C (x,y) at block 42 from which the nowadjusted image B′ C (x,y) is directed to the printer controller.

FIG. 3 presents the process steps in the described embodiment. In stepS110, the potential ink coverage in separation N of B(x,y) is measured.In Step S130, the location in the image of pixels that should be turnedOFF is determined in accordance with the turn OFF ratio. Step S140compares each separation with the others to prevent corresponding pixelsin each separation from being all turned OFF. Next, step S155 setspixels in the separation of B(x,y) OFF. Finally, in step S160, theadjusted image is output to a print controller.

FIG. 4 demonstrates a flow chart for a single pass across an image,ignoring image edges, in which the method of ink coverage measurement isconducted. This flow chart is demonstrated for the cyan bitmap B C(x,y), but it will be understood that the process is the same for theother color bitmaps. First, at step S200, the scan line and pixel valuesare initialized to a starting value (here, 0) for the cyan bitmap B C(x,y). At step S210, a first subset of pixels (here, n) from a firstscan line at latch 12 is stored. Step S220 shows using the stored n bitvalue to access a memory location in LUT 14. An ink coverage value I C(y) is retrieved at step S230 for the set of pixels, which value I C (y)is stored in step S240 in memory 20. At step S250, the scan line valueis incremented so that the process may be repeated for the next subsetof pixels from a second scan line. If a desired area of coverage, herean 8 scan line region, has been examined, the test of step S260 is met,the scan line value is reset to 0 and at step S270, the pixel value isset for the next subset of pixels in each scan line. If the test at stepS260 is met, the ink coverage value p for the 8×8 area in the separationis determined by a standard summing function, indicated by the equationof step S280. At step S290, p_(c), p_(m), p_(y), and p_(k) are summed toderive a value of m for the given area of the image, and the valuesp_(c), p_(m), and p_(y) are summed to derive a value of m_(c) for thegiven area of the image.

FIG. 5, shows, as before, a flow chart for a single pass across animage, ignoring image edges, to demonstrate the pixel ON/OFFdetermination. The determination process includes a first step S300 inwhich d is set, probably entered by a user of the system as required bythe combination of ink and material. At step S310, the scan line andpixel values are initialized to starting values (here, 0). Next, at stepS320, m, m_(c), and m_(k) are determined for a particular 4×4 subareawithin the 8×8 area for which m, m_(c), and m_(k) were determined, usingthe ink coverage measurement m determined by the method described withrespect to FIG. 4. Steps S340 and S350 together store the pixel valuesof B C (x,y) in a memory 25. At step S360 pixels are read out of memory25 by reorder processor 26 in order F C (x,y), to avoid moiré. F C (x,y)may vary from separation to separation, if required to avoid moiré.

In FIG. 5, white pixel prevention logic is combined with the ON/OFFlogic. At step S370, bitmap B C (x,y) is examined to determine whetherthe current pixel is ON or OFF. If it is OFF, the pixel remains OFF(step S372), and the next pixel is processed (step S374). If bitmap B C(x,y) is examined at step S370 and the current pixel is ON, at stepS376, f is incremented by the value d_(c). At step S378, the “count” iscompared to 1. The count represents a running sum of other separationsthat are turned ON at the current x,y position. If the count equals 1,then only the current separation is turned ON, and accordingly, to avoida white space, the current pixel should be turned ON at step S380. Ifthe count is greater than 1, then there will be no white space occurringanyway. At step S382, f is compared to m. If f is less than or equal tom, then the pixels remains ON, at step S380. If f is not less than orequal to m, then f is decremented by m at step S384, and the pixel isturned OFF at step S372.

Alternatively, white pixel prevention logic need not be combined withthe ON/OFF logic. Such an embodiment is demonstrated in FIG. 6, whichshows a flow chart showing separate white pixel prevention logic. InFIG. 6, if B C ON/OFF (x,y) is OFF (step S410), then each separationturn off function B N ON/OFF (x,y) (represented for the purpose of thisfigure as B c, B m, B y, and B k) is ANDed with the others (step S420)to derive cm, cy, ck, my, ink, yk, which are then ORed together toderive a determination SEP ON/OFF. If SEP ON/OFF is greater than 0 (StepS440), then at least one of the separations is to be turned ON, and noaction is required. B C ON/OFF (x,y) is directed to an output at stepS450. However if SEP ON/OFF is equal to 0 (Step S440), then B ON/OFF(x,y) is turned ON. This is a rather simplistic solution, sinceundoubtedly, the choice of separation that is turned ON in white pixelprevention should be alternated among the separations that were turnedOFF. Otherwise, if, for example, the cyan separation is always processedfirst, then the image may be skewed with a greater amount of cyan thandesired.

While the present invention is demonstrated with the above embodiment,there is no doubt many others accomplish the invention. The importantaspects of the invention are that ink coverage can be measured byreference to the bitmap of the image, and that by a comparison ofmeasured ink coverage with desired ink coverage, an ON/OFF ratio can bedetermined and applied to the pixels in the color separations of theimage, retaining the black color separation unchanged, to reduce inkcoverage. Preferably, the determination is made in a manner thatminimizes moire, and preferably, makes the determination of OFF or ON inany single separation with knowledge about the state of thecorresponding pixels in other separations. The present descriptionassumes a binary or bilevel ink jet printer, but there is no reason inprinciple that the invention would not apply to a gray or othermultilevel printer, where ink coverage is a function not only of thepresence of a print dot, but also the gray level value of the print dot.Other printing processes that are printed on a pixel by pixel basis alsohave problems with excessive marking material, and may benefit from thedescribed method of reduced marking material use. Notably,electrophotographic and ionographic printing methods using a pixel bypixel printing process for forming a latent image for development withdry or liquid toner marking materials, can suffer from excessive markingmaterial, evidenced by sheet cockling and curling caused by differentialshrinkage of toner and paper in the printing process. Thus, although thepresent invention has been described with reference to an ink jetprinting system, there is no reason that the present invention can notbe applied to such other printing processes, and application of thepresent invention in such non-ink jet printing processes is fully withinthe scope of the present invention.

An advantage of the present invention is that the resultant image moreclosely matches the original image being processed. In usual coveragereduction techniques, such as that disclosed in U.S. Pat. No. 5,515,479,all of the color separations are processed to reduce marking materialused by all of the separations. Thus, once the desired amount of markingmaterial, d, is determined, all of the color separations, including theblack color separations, are reduced by the reduction ratio d/m. In thismanner, the reduction ratio applied to all of the separations is thesame. However, according to the present invention, the reduction ratioof black is set to one, and a smaller reduction ratio is applied to theremaining color separations. Thus, the ratio of any color separation toblack is allowed to float (depending on the relative weighting of blackin the image), while the ratios of other color separations to each otheris maintained constant. It has been found that this process results inmore realistic color rendition, providing resultant images without colorshift problems.

As will be apparent to one of ordinary skill in the art, numerouschanges, alterations and adjustments can be made to the above-describedembodiments without departing from the scope of the invention, and theinvention is in no way limited to the specific exemplary embodimentsdescribed above. One skilled in the art will recognize that the variousaspects of the invention discussed above may be selected and adjusted asnecessary to achieve specific results for a particular printerapplication. Thus, the foregoing embodiments are intended to illustrateand not limit the present invention. It will be apparent that variousmodifications can be made without departing from the spirit and scope ofthe invention.

What is claimed is:
 1. A method for processing a color image forprinting to reduce an amount of marking material used therefor, thecolor image comprised of multiple separations, comprising at least onenon-black color separation and a black color separation, each defined bya bitmap of single bit pixels each having a pixel value and arranged inan array of scan lines, said method comprising the steps of: for eachseparation bitmap, measuring a marking material coverage value over agiven area within the image; summing the marking material coverage ofeach separation bitmap to determine measured marking material coverage;comparing desired marking material coverage with measured total markingmaterial coverage to derive an ON/OFF ratio, where the ON/OFF ratiorepresents a number of pixels to be turned OFF, compared to the numberof pixels ON in a separation; setting a number of pixels to OFF in thenon-black color separation bitmaps, in a larger number of pixels in saidseparation, in accordance with the turn OFF ratio; retaining an ON/OFFstate of all of the pixels in the black color separation irrespective ofthe ON/OFF ratio; and outputting the separation bitmaps to a printcontroller.
 2. The method as defined in claim 1, wherein the step ofmeasuring marking material coverage comprises the steps of: directingpixel values of a set of k pixels from a single scan line of pixels intoa latch, where k is an integer number of pixels; providing a pre-loadedlook up table, having a set of k bit addresses, each addresscorresponding to data entries including a possible combination ofmarking material coverage values for a set of k pixels; storing to amemory, the marking material coverage values from the look up tablecorresponding to each set of k pixel values for a set of e scan lines;and summing for the image, the marking material coverage values for eachseparation over the area of a set of k pixel values in a set of e scanlines.
 3. The method as defined in claim 1, wherein the step of settinga number of pixels to OFF in the bitmap, in a larger number of pixels inthe non-black color separation in accordance with the turn OFF ratiocomprises the steps of: incrementing a counter value by a predeterminedamount related to the OFF/ON ratio, whereby increasing the counter valueincreases the likelihood of the pixel being set OFF; comparing thecounter value with a reference to determine whether to set the pixelOFF; upon determining that a pixel should be turned OFF, resetting thecounter so that the next pixel has a decreased probability of beingturned OFF; and performing a masking process between the pixel value andthe ON/OFF determination to generate the separation bitmap.
 4. Themethod as defined in claim 1, wherein the step of comparing desiredmarking material coverage with measured marking material coverage toderive a turn OFF ratio, where the turn OFF ratio represents a number ofpixels to be turned OFF, in a larger number of pixels in a separationfurther comprises the step of determining the location in the image ofpixels that should be turned off in accordance with the turn off ratio.5. The method as defined in claim 4, wherein the step of determining thelocation in the image of pixels that should be turned off in accordancewith the turn off ratio further comprises the step of processing thepixels within the given area in a predetermined order.
 6. The method asdefined in claim 5, wherein the predetermined order corresponds to thepath of a space filling function through each pixel in the given area.7. The method as defined in claim 4, wherein the step of determining thelocation in the image of pixels that should be turned OFF in accordancewith the turn off ratio further comprises the step of comparing thestate of corresponding pixels in each separation bitmap and upondetection of a concurrence of pixels turned OFF, turning at least onepixel from said corresponding pixels, ON.
 8. A method for processing acolor image for printing by an ink jet printer to reduce ink usedthereby, the color image comprised of multiple separations, comprisingat least one non-black color separation and a black color separation,each defined by a bitmap of single bit pixels each having a pixel valueand arranged in an array of scan lines, comprising the steps of: foreach separation bitmap, measuring an ink coverage value over a givenarea within the image and summing the ink coverage of each bitmap todetermine total ink coverage; comparing desired ink coverage withmeasured total ink coverage to derive an ON/OFF ratio, where the ON/OFFratio represents a number of pixels to be turned OFF, compared to thenumber of pixels ON in a separation; setting a number of pixels to OFFin the non-black color separation bitmaps in a larger number of pixelsin a separation, in accordance with the turn OFF ratio; retaining anON/OFF state of all of the pixels in the black color separationirrespective of the ON/OFF ratio; and outputting the separation bitmapsto a print controller.
 9. The method as defined in claim 8, wherein thestep of measuring ink coverage comprises the steps of: directing pixelvalues of a set of k pixels from a single scan line of pixels into alatch, where k is an integer number of pixels; providing a pre-loadedlook up table, having a set of k bit addresses, each addresscorresponding to data entries including a possible combination of inkcoverage values for a set of k pixels; storing to a memory, the inkcoverage values from the look up table corresponding to each set of kpixel values for a set of 1 scan lines; and summing for the image, theink coverage values for each separation over the area of a set of kpixel values in a set of 1 scan lines.
 10. The method as defined inclaim 8, wherein the step of setting a number of pixels to OFF in thebitmap, irrespective of their state, in a larger number of pixels in thenon-black color separation in accordance with the turn OFF ratiocomprises the steps of: incrementing a counter value by a predeterminedamount related to the OFF/ON ratio, whereby increasing the counter valueincreases the likelihood of the pixel being set OFF; comparing thecounter value with a reference to determine whether to set the pixelOFF; upon determining that a pixel should be turned OFF, resetting thecounter so that the next pixel has a decreased probability of beingturned OFF; and performing a masking process between the pixel value andthe ON/OFF determination to generate the separation bitmap.
 11. Themethod as defined in claim 8, wherein the step of comparing desired inkcoverage with measured ink coverage to derive a turn OFF ratio, wherethe turn OFF ratio represents a number of pixels to be turned OFF, in alarger number of pixels in a separation further comprises the step ofdetermining the location in the image of pixels that should be turnedoff in accordance with the turn off ratio.
 12. The method as defined inclaim 11, wherein the step of determining the location in the image ofpixels that should be turned off in accordance with the turn off ratiocomprises the step of processing the pixels within the given area in apredetermined order.
 13. The method as defined in claim 12, wherein thepredetermined order corresponds to the path of a space filling functionthrough each pixel in the given area.
 14. The method as defined in claim11, wherein the step of determining the location in the image of pixelsthat should be turned OFF in accordance with the turn off ratio furthercomprises the step of comparing the state of corresponding pixels ineach separation bitmap and upon detection of a concurrence of pixelsturned OFF, turning at lease one pixel from said corresponding pixels,ON.
 15. A method for processing a color image for printing, to reduce anamount of marking material used thereby, the color image comprised ofmultiple separations, comprising at least one non-black color separationand a black color separation, each defined by a bitmap of single bitpixels each having a pixel value and arranged in an array of scan lines,comprising the steps of: receiving pixel values of a set of k pixelsfrom a single scan line of pixels, where k is an integer number ofpixels; providing a pre-loaded look up table, having a set of k bitaddresses, each address corresponding to data entries including apossible combination of marking material coverage values for a set of kpixels; storing to a memory, the marking material coverage value fromthe look up table corresponding to each set of k pixel values for a setof 1 scan lines; summing the marking material coverage values for theset of 1 scan lines to derive a marking material coverage value for thearea of k pixels by 1 scan lines in each separation; summing the markingmaterial coverage values for the separations to derive a markingmaterial coverage value for the image over an area of k pixels by 1 scanlines; determining the location in the image of pixels in the non-blackcolor separations that should be turned OFF in accordance with a turnOFF ratio, while retaining unchanged an ON/OFF state of all pixels inthe black color separation; incrementing a counter value by apredetermined amount related to the turn OFF ratio, whereby increasingthe counter value increases the likelihood of the pixel being set OFF;comparing the counter value with a reference to determine whether to setthe pixel OFF, said comparing generating an ON/OFF determination signal;upon determining that a pixel should be turned OFF, resetting thecounter so that the next pixel has a decreased probability of beingturned OFF; performing a masking process between the pixel value and theON/OFF determination signal to generate a separation bitmap; andoutputting each separation bitmap to a printer.
 16. The method asdefined in claim 15, wherein the step of determining the location in theimage of pixels that should be turned off in accordance with the turnoff ratio comprises the step of processing the pixels within the area ina predetermined order.
 17. The method as defined in claim 16, whereinthe predetermined order corresponds to the path of a space fillingfunction through each pixel in the area.
 18. The method as defined inclaim 15, wherein the step of determining the location in the image ofpixels that should be turned off in accordance with the turn off ratiofurther comprises the step of comparing the state of correspondingpixels in each separation bitmap and upon detection of a concurrence ofpixels turned OFF, turning at least one pixel from said correspondingpixels ON.
 19. A device for processing a color image comprised ofmultiple separations, comprising at least one non-black color separationand a black color separation, each defined by a bitmap of single bitpixels for printing, to reduce an amount of marking material used in thecolor image, comprising: means for predicting the marking materialcoverage of an area in an output image as a function of the input image;means for determining a ratio of pixels to be turned OFF in thenon-black color separations, while retaining unchanged an ON/OFF stateof all pixels in the black color separation, as a function of a desiredmarking material coverage for a page on which the color image is printedand the predicted marking material coverage; processing order controlmeans, for determining a sequence of pixels processed in the input imagefor reducing marking material coverage; turn off determining means fordetermining whether a pixel should be turned OFF in the non-black colorseparations in accordance with the turn OFF ratio, where each pixel isprocessed in an order controlled by the process order control means,wherein said turn off determining means does not operate on said blackcolor separation; a masking circuit, having as inputs the pixels in eachseparation of the color image and the turn off ratio determination fromsaid turn off determining means, and having as an output a pixel valuefor a reduced marking material image; and output means for directing thepixel value to a print controller.
 20. The device as defined in claim19, wherein the turn off determining means for determining whether apixel should be turned OFF in accordance with the turn OFF ratiocomprises: a counter, maintaining a count value incremented by apredetermined amount related to the turn OFF ratio for each pixelprocessed, whereby increasing the counter value increases the likelihoodof the pixel being set OFF; means for comparing the count value with areference to determine whether to set the pixel OFF; and means forresetting the counter so that the next pixel has a decreased probabilityof being turned OFF.
 21. The device as defined in claim 19, wherein theturn off determining means for determining whether a pixel should beturned OFF in accordance with the turn OFF ratio comprises: means forcomparing the state of corresponding pixels in each separation bitmapand upon detection of a concurrence of pixels turned OFF, turning atleast one pixel from said corresponding pixels, ON.